In general, structures resist earthquake attack by a combination of strength, (that is, ability to withstand large forces while remaining elastic), deformability, and energy-absorbing capacity. In initial approach in aseismic design placed emphasis on the strength aspect and increased strength was provided to resist horizontal earthquake forces. However, it was eventually shown that such structures are deformed well beyond their elastic limit during severe earthquakes and that the structures survived because they were capable of deforming inelastically, which gave both increased flexibility and energy absorption.
The principal approach to aseismic design then became the design of structures with a large capacity for plastic deformation so that strength requirements could be reduced, thereby leading to a reduction in structural cost without any loss in earthquake resistance. The approach remains as the one most used for high-rise construction today, but it suffers from several disadvantages:
(i) Progressive deterioration of the structural components can occur under the cyclic loading produced by a strong-motion earthquake. PA1 (ii) Large relative movements are necessary to achieve significant energy absorption and for example, building facings, partitions and services may be damaged. PA1 (iii) Such high-strength structural forms as shear walls or shear towers, frames with deep beams and wide columns, stiff peripheral frames, diagonally-braced space frames and shells and diaphragms, which can be built at relatively low cost, must be excluded because they have little capacity for plastic deformation. PA1 at least one support having flexibility in a horizontal direction of motion of at least a major portion of the structure; and PA1 at least one hysteretic energy absorber capable of absorbing horizontal accelerator forces originating from earthquakes and consequential motions, said hysteretic energy absorber being adapted so as not to be actuable by normal ambient accelerating forces. PA1 (a) isolating a foundation from the structure resting thereon by allowing for limited relative movement in a horizontal direction between said foundation and said structure; PA1 (b) absorbing horizontal accelerating forces originating from earthquakes and consequential forces imparted to said structure by means of an hysteretic energy absorber; and, PA1 (c) withstanding normal ambient accelerating forces imparted to said structure by means of said hysteretic energy absorber.
If the earthquake-generated forces could be reduced without the need for plastic deformation, these low-cost forms of construction could be employed. Rather than relying on plastic deformation of the structure components, which leads to damage of the structure, replaceable energy-absorbing devices could be distributed throughout the structure.
This procedure is adopted for some tall buildings in Japan, in which energy absorption takes place in reinforced concrete panels, which are provided with slits to give regions of hysteretic absorption of energy. Alternative absorbers, utilizing the hysteretic deformation of steel beams are described and claimed in U.S. Pat. Nos. 3,831,924 issued Aug. 27, 1974 entitled "Tortional Energy Absorber" and 3,963,099 issued on June 16, 1976, entitled "Hysteretic Energy Absorber." Any of the forms of damper described or claimed in those patents may be used. However, if the absorbers are incorporated in the structure they are effective only during substantial deformations of the structure and therefore there is still a likelihood of considerable secondary damage.
Recent earthquakes have demonstrated that all these techniques adopted to protect structures, structural functions, and non-structural components from impairment by earthquake attack are only partially successful.
Most of the problems associated with current methods for providing earthquake resistance would be removed if the energy absorption and flexibility required to reduce earthquake generated forces could be provided by a system which is independent of the structural components. This concept leads immediately to the idea of a mechanical attenuator between the structure and the foundation on which it stands.
Attenuators, to be placed between equipment and the base on which it stands, to reduce forces arising from impulsive of cyclic motion of the base are well known. These usually consist of a flexible system combined with a damping system. Heretofore there has been no practical proposal for a combination of a flexible system together with a damping system adapted for use in an earthquake attenuator for structure.
It is therefore an object of the present invention to provide a mechanical attenuator that will go some way to supply this need without the drawbacks aforesaid, or will at least provide the public with a useful choice.
The hysteretic dampers aforementioned are practical and low-cost dampers which may be used in such attenuators. There exist a number of practical base-support systems which allow a more or less free undamped, horizontal motion of the base of a structure. A combination of such a base support system and the recently developed hysteretic energy absorbers constitutes a practical earthquake isolation system of mechanical attenuator for structures.